Typically in the method of testing gas meters, air from a proving bell is passed through a meter until a unit volume is measured by the meter. The proving bell is stopped and the calibration is accomplished by comparing the known volumn passed from the proving bell to the meter reading. The meter accuracy, expressed as Percent Proof, is the ratio of the actual volume indicated by the position of the prover bell to the registered volume shown on the meter index. Generally the calibration is performed at 100 percent of rated meter flow and at 20 percent of the rated meter flow.
Prior art relating to this method of testing gas meters, utilize various mechanical attachments to the prover bell and/or meter to provide an indication of the small incremental volumes by which the prover bell and meter differ when the proving bell is stopped. The attachments to the meter and bell actuate various electro-mechanical circuits to operate solenoid valves to take the prover bell and meter through a proving cycle. The more sophisticated art involves the attachment of a potentiometer to the prover bell wheel shaft to provide an analog signal indicative of bell position. The mechanical attachment of the potentiometer to the prover wheel shaft has been shown to develop a mechanical set making it difficult to mechanically adjust. The analog circuitry associated with the potentiometer has been generally shown to have a tendency to drift electrically because of aging and/or temperature changes so that their accuracy is poor, requiring frequent calibration.
It is commonly accepted in the proving of meters that for meters operating at a constant rate of flow, time may be used to indicate the volume that the meter is measuring. The time required to measure a unit volume of a prover bell operation at a constant rate of flow, and the time to measure a unit volume of a meter connected to the prover bell may be substituted for volume in the calibration equation: ##EQU1##